This invention relates to molten aluminum treatments and more particularly it relates to molten metal treatment for degassing and/or forming grain refining nuclei in molten aluminum in situ.
There is an ever increasing effort to improve aluminum and its alloys by the use of new grain refiners or master alloys comprising the grain refiners. Presently, the most popular grain refiners for aluminum utilize titanium diboride (TiB.sub.2) type compound [(TiAl)B.sub.2 ] or titanium carbide (TiC). Typically, the TiB.sub.2 is produced by reacting K.sub.2 TiF.sub.6 and KFB.sub.4 salts with aluminum to produce the master alloy with excess titanium. The master alloy is added to the molten aluminum to be refined prior to the casting operation and usually prior to filtration. This master alloy manufacturing process produces small particle sized TiB.sub.2 and TiAl.sub.3, entrained KAlF and Al.sub.2 O.sub.3. Sometimes the salts are added directly to the molten metal to be refined. When the master alloy is used, it is added to the melt as a waffle or rod.
However, the use of master alloys or addition of the salts directly to the metal is not without problems. For example, the master alloys containing TiB.sub.2 often have salt and oxide inclusions, e.g., titanium and boron salts and aluminum oxides. Often the inclusions are larger than the TiB.sub.2 particles. Further, the master alloy also can contain TiB.sub.2 clusters or large TiB.sub.2 particles which, of course, again are larger than the individual TiB.sub.2 particles. The TiB.sub.2 clusters normally contain materials such as oxides and salts, e.g., KAlF.sub.4. The inclusions and clusters are detrimental because they are frequently the source of downstream processing problems in the cast or fabricated aluminum product. For example, the inclusions and clusters cause increased wear on cutting, rolling or die surfaces used to process the cast aluminum product. The inclusions and clusters are a source of defects such as holes and stress points in the metal. Further, the inclusions and clusters are detrimental because of filter clogging just prior to the casting operation, adding an additional expense in filter replacement.
Another very effective grain refiner that is being increasingly used with aluminum is Al-3% Ti-0.15C. Master alloys containing TiC are often prepared by heating mixtures of aluminum, titanium and carbon. However, this method has the problem that the process temperature is quite high, e.g., 1200.degree. to 1300.degree. C. Further, the process only makes a dilute master alloy, i.e. a master alloy that is dilute in grain refining nuclei, e.g., Al-3% Ti-0.15C. With this grain refining system, there are problems with purity resulting from oxide inclusions and carbon cores.
In addition, the use of master alloys has the problem that they provide localized, high concentrations of refiner which can result in larger clusters of particles, residual slag (KAlF.sub.4), oxides, etc., and inoperative nuclei as well as problems dispersing the particles and dissolving the aluminides. Even though master alloys are presently used throughout the industry, they are an inefficient use of the refiner components. The high concentrations of refiner referred to are even more pronounced in foundry situations where cast waffles of master alloys are added to the aluminum melt.
The use of the term "aluminum" as used herein is meant to include aluminum and its alloys.
Prior attempts at improving grain refining have focused on improving the master alloy. For example U.S. Pat. No. 5,415,708 discloses an aluminum base alloy consisting essentially of from 0.1 to 3.0% boron, from 1 to 10% titanium and the balance essentially aluminum wherein the aluminum matrix contains TiB.sub.2 particles dispersed throughout said matrix having an average particle size of less than 1 micron, and wherein the matrix contains clusters of said TiB.sub.2 particles greater than 10 microns in size with an average of less than 4 of said clusters per 2 cm.sup.2. The alloy is prepared by adding a boron containing material selected from the group consisting of borax, boron oxide, boric acid and mixtures thereof, and K.sub.2 TiF.sub.6 to a bath of molten aluminum and stirring the molten mixture.
U.S. Pat. No. 5,100,618 discloses a process for producing aluminum grain refiner, such as Al--Ti--B grain refiner. Molten aluminum is continuously flowed as a bottom layer along a substantially horizontal or slightly inclined trough. Titanium or boron compounds reducible by aluminum or a mixture of such compounds is added to the surface of the aluminum layer such that a discrete separate layer of these is formed on top of the aluminum layer. Reaction between the aluminum and the titanium and/or boron compounds occurs along the interface between the layers and this reaction may, if desired, be aided by providing relative movement between the layer of molten aluminum and the layer of titanium and/or boron compounds.
U.S. Pat. No. 5,104,616 discloses a method for the production of master alloys intended for grain refining of aluminum melts and being of the type which comprises of aluminum and 1-15 percent by weight titanium, where titanium is present in the form of intermetallic crystals of titanium aluminide in combination with additives of carbon and/or nitrogen. The method is characterized by adding carbon and/or nitrogen to the aluminum melt in an amount corresponding to at least 0.01 percent by weight in the resultant solidified material. The addition of the carbon and/or nitrogen is effected in elemental form or in the form of dissociable carbon and/or nitrogen containing compounds, making said addition before or during an established thermodynamic state of dissolution of existing crystals of titanium aluminide, and bringing the melt into a thermodynamic state where crystals of titanium aluminide present grow in size and thereafter causing the melt to solidify.
U.S. Pat. No. 3,961,995 discloses an aluminum-titanium-boron mother alloy having a boron content of 0.2 to 0.8% by weight and a titanium content such that Ti--2.2 B.gtoreq.3.9%, in which the matrix has a preponderant proportion of grains of less than 30 microns in size, and contains fine TiB.sub.2 crystals having an average size of about 1 micron primarily dispersed along the grain boundaries, and the method for the preparation of same by the formation of titanium diboride by the action of liquid aluminum on titanium oxide and boron oxide in solution in molten cryolite, mixing the reactants in a manner to utilize the starting materials, and then quenching the formed alloy rapidly to cool and solidify the mother alloy, preferably by pouring the liquid alloy in water to produce the alloy in the form of granules or fine powder.
U.S. Pat. No. 4,803,372 discloses a process for producing a composite comprising a refractory material dispersed in a solid matrix. A molten composition comprising a matrix liquid, and at least one refractory carbide-forming component are provided, and a gas is introduced into the molten composition. A reactive component is also provided for reaction with the refractory material-forming component. The refractory material-forming component and reactive component react to form a refractory material dispersed in the matrix liquid, and the liquid composite is cooled to form a solid composite material.
British Patent 1,333,957 discloses a method of preparing a master alloy intended to be added to an aluminum melt to control the grain size during solidification thereof, which comprises providing a master alloy melt containing aluminum together with 0.02 to 6% by weight of titanium and 0.01 to 2% by weight of boron and holding the master alloy melt at a temperature between its melting point and 900.degree. C. under agitation for a period of at least 15 minutes and at most 9 hours.
U.S. Pat. No. 5,100,488 discloses an improved aluminum-titanium master alloy which contains in weight percent, carbon about 0.005 up to 0.05 titanium 2 to 15, and the balance aluminum. After melting, the master alloy is superheated to about 1200.degree. C.-1300.degree. C. to put the element into solution, then the alloy is cast in a workable form. The master alloy in final form is substantially free of carbides, sulfides, phosphides, nitrides, or borides greater than about 5 microns in diameter. The alloy of this invention is used to refine aluminum products that may be rolled into thin sheet, foil, or fine wire and the like. Such grain refined products are also substantially free of carbides, sulfides, phosphides, nitrides or borides.
U.S. Pat. Nos. 5,041,263 and 4,812,290 disclose an improved aluminum-titanium master alloy containing carbon in an amount not more than about 0.1%. After melting, the master alloy is superheated to about 1200.degree. C.-1250.degree. C. to put the carbon into solution, then the alloy is cast in a workable form. The master alloy in final form is substantially free of carbides greater than about 5 microns in diameter. The alloy of this invention is used to refine aluminum products that may be rolled into thin sheet, foil, or fine wire and the like.
U.S. Pat. No. 4,556,419 discloses hydrogen gas and non-metallic inclusions removed from molten aluminum by a process comprising the steps of maintaining an atmosphere containing BF.sub.3 gas in a treating vessel above the surface of molten aluminum placed therein, introducing a treating gas into the molten aluminum, and removing floating non-metallic inclusions and treating gas containing hydrogen gas from the surface of the molten aluminum.
U.S. Pat. No. 4,873,054 discloses an improved aluminum-titanium master alloy. Such alloy contains a small but effective amount of, in weight percent, any two or more elements selected from the group consisting of carbon about 0.003 up to 0.1, sulfur about 0.03 up to 2, phosphorus about 0.03 up to 2, nitrogen about 0.03 up to 2, and boron about 0.01 up to 0.4, titanium 2 to 15, and the balance aluminum.
U.S. Pat. Nos. 4,842,821 and 4,748,001 disclose a method of producing an alloy containing titanium carbide particles, the method comprising thoroughly dispersing carbon powder particles into a metal melt, and causing the dispersed carbon particles to react with titanium within the metal melt so as to produce a dispersion of fine particles comprising titanium carbide within the melt. A preferred use for alloys produced by the invention is as a grain refiner for aluminum-based metals, especially those containing zirconium, chromium and/or manganese, which tend to poison current titanium-boron-aluminum grain refiners.
U.S. Pat. No. 4,392,888 discloses molten aluminum or other metals purified by contacting with a fluorocarbon, such as CCl.sub.2 F.sub.2, in order to decrease the amount of impurity metal elements along with gas and inclusions therein preferably in the presence of an agitator to enhance efficiency. An oxidizer, such as oxygen, is employed to prevent the carbon in the fluorocarbon from forming carbide inclusions. Oxidizing the carbon to carbon monoxide is preferred in treating aluminum since the monoxide effectively removes the carbon from the system without oxidizing aluminum.
German Patent 1,027,407 discloses a process for grain refining aluminum alloys, especially magnesium containing alloys which contain boron and/or titanium characterized by the fact that carbon is produced in finely divided form by decomposing carbon compounds, e.g., carbon tetrachloride, which are introduced to the melt in a carrier gas to promote formation of carbides as crystallization nuclei. The patent states that the carbon tetrachloride is not dangerous to foundry personnel because its vapor will decompose at normal casting temperatures.
U.S. Pat. No. 4,402,741 discloses a process and an apparatus for the precise and continuous injection of a halogenated derivative, which is liquid at ambient temperature, into a liquid metal such as aluminum and aluminum-based alloys. The process involves withdrawing the halogenated substance from a tank, introducing it by means of a metering pump into a vaporizer which has been brought to a temperature at least equal to the vaporization temperature of the substance under the injection pressure, and entraining it in the vapor state by an inert gas stream towards an injection means opening into the center of the liquid metal.
In spite of these disclosures there is still a great need for improvements in grain refining of aluminum which do not contaminate the metal and which permit grain refining at molten aluminum processing temperatures. The present invention provides such an improvement.